The investigator proposes to continue study of the pathophysiologic mechanisms of altered states of calcium and phosphate excretion. These studies will include evaluation of renal tubular defects in patients with Vitamin D resistant rickets and clearance and micropuncture studies in mice with an identical hereditary disease. The tubular sites responsible for the regulation of urinary calcium excretion will be determined using a variety of techniques. Micropuncture studies will be performed in rats previously made hypercalciuric by depletion of dietary phosphate. Microperfusion of isolated tubular segments from the kidneys or rabbits also subjected to phosphate depletion will be utilized to more precisely characterize the site and mechanisms responsible for increased calcium excretion.